Recently, nitride semiconductor laser devices have been receiving increasingly demands for the applications in optical disk systems such as DVD which are capable of recording and reproducing a large amount of information with a high density. Accordingly, vigorous research efforts are being made in the field of nitride semiconductor laser device. Because of the capability to oscillate and emit visible light over a broad spectrum ranging from ultraviolet to red, the nitride semiconductor laser device is expected to have wide applications such as light sources for laser printer and optical network, as well as the optical disk system.
With regard to the structure of the laser device, in particular, various researches have been made and a number of proposals have been made for the structure that enables preferable control of transverse oscillation mode. Among these, ridge waveguide structure is viewed as promising, and is employed in the nitride semiconductor laser device that was shipped first in the world.
The ridge waveguide structure for a semiconductor laser device makes it easier to drive laser oscillation due to the simple structure, although variations are likely to occur in the characteristics of the devices during volume production. This is because the characteristics are caused to vary by variations in the dimensions of mesa stripe in the case of ridge waveguide structure, while the dimensional accuracy of the mesa stripe is determined by the accuracy of etching and the dimensional accuracy of the mesa stripe cannot be made higher than the accuracy of etching. In the case of a semiconductor laser device made from a semiconductor material which is likely to suffer significant etching damage in the active layer or damage caused by exposing the active layer surface to the etching atmosphere, laser characteristics degrade due to the etching damage in the active layer and on the active layer surface when a semiconductor laser device of perfect refractive index guided type is made by etching deeper than the active layer thereby forming ridges. Therefore, such a semiconductor laser device must be made in the effective refractive index type waveguide structure wherein stripes are formed to a depth that does not reach the active layer. However, in the case of effective refractive index type waveguide structure, variations in the device characteristics due to the variation in the stripe configuration mentioned above become significant, thus resulting in considerable variations in the device characteristics during volume production.
In order to apply the nitride semiconductor laser device in the fields described previously, it is indispensable to provide a device which can be mass-produced with stable quality.
However, the structure of the laser devices known at present has a bottle neck in the formation of the ridge waveguide. This is because, while the ridge waveguide is formed by growing nitride semiconductor that constitutes the device, then removing a part of the nitride semiconductor by etching the upper layer thereby forming the ridge which constitutes the waveguide, accuracy of the etching has a great effect on the characteristics of the laser device obtained as mentioned previously. That is, since the transverse mode is controlled by the configuration, particularly height and width, of the ridge that constitutes the ridge waveguide and the far field pattern (F.F.P.) of the laser beam is determined accordingly, an error in the control of the depth of etching when forming the ridge waveguide is a major factor which directly causes variations in the device characteristics.
Dry etching techniques such as reactive ion etching (RIE) have been known for etching nitride semiconductor, but it has been difficult to control the depth of etching to such an accuracy as to completely solve the problem of variations in the device characteristics with these etching techniques.
Design of devices in recent years in a trend to have a multitude of layers that are controlled to be several atoms in thickness formed in the device, such as in the case of super lattice structure. This also contributes to the variations in the device characteristics caused by the etching accuracy. Specifically, when forming the layers which constitute the device structure, the layers are formed with an extremely high accuracy and it is difficult to achieve the device structure of sophisticated design by forming the ridge and other structure with the etching technique having an accuracy lower than the accuracy of film forming by several orders of magnitude, thus making an obstacle to the improvement of device characteristics.
For example, when forming a nitride semiconductor laser device having a high output power in the refractive-index guiding type structure where ridge waveguide is provided on the active layer without etching the active layer, accuracy of etching depth must be controlled so as to keep the effective difference in refractive index between a portion of the active layer right below the ridge and other portion of the active layer to one hundredth. In order to achieve this accuracy, the ridge must be formed by etching while controlling the depth with an accuracy within 0.01 μm till a very small portion of p-type cladding layer remains, in case the layer right above the active layer is the p-type cladding layer. On the other hand, width of the ridge waveguide may have lower accuracy but must be etched with an accuracy of 0.1 μm.
When the RIE process is employed for etching the nitride semiconductor, the layer exposed by etching and the surface thereof are prone to damage, which leads to deterioration of the device characteristics and reliability. Etching can be done in a wet etching process as well as a dry etching process, although wet etching solution which is applicable to nitride semiconductors has not been developed.
As described above, whether nitride semiconductor laser device having high functionality can be made or not in volume production with less variations in the characteristics heavily depends on the accuracy of forming the ridge waveguide in the etching process, and it has critical importance to form the ridge waveguide with a high accuracy.
In light of the circumstances described above, the present inventors have invented a laser device or an end face light emission device and a method for manufacturing the same which, even in the case of a semiconductor laser device of stripe configuration and despite the semiconductor laser device has a resonator of excellent oscillation and wave guiding characteristics, allows stable control of transverse mode and is capable of emitting laser beam of excellent F.F.P., with less variations in the device characteristics even when mass-produced.